The present invention relates to a rotation detecting device, and more particularly to a rotation detecting device adapted to be mounted to a transmission or the like of an automobile for converting rotation of the transmission to an electric signal and outputting the electric signal to an indicating instrument or the like.
Conventionally, a semiconductor type rotation detecting device is often used as a vehicle speed sensor to be mounted on an automobile. The semiconductor type rotation detecting device includes a magnetic induction device and a magnet located near the magnetic induction device. Rotation of a transmission of an automobile is transmitted to the magnet, and a change in magnetic field due to the rotation of the magnet is detected by the magnetic induction device. An electric signal generated from the magnetic induction device is output to an indicating instrument or the like.
FIG. 16 shows such a prior art rotation detecting device. Referring to FIG. 16, a rotation detecting device 401 includes a cylindrical housing 402 in which rotating portion 403 and current feeding portion 404 are installed as a unit.
A rotor 405 is rotatably provided in the rotating portion 403. An outer circumferential surface 405a of the rotor 405 is magnetized to have multiple poles. An input rotating shaft 407 is rotatably supported to a bearing 418, and is fixedly engaged with a central portion of the rotor 405. The input rotating shaft 407 is formed with a hollow portion 408. A connecting portion 460 is formed at one end of a coupling 409 to be connected to a rotational driving source of a transmission (not shown), and the connecting portion 460 is inserted in the hollow portion 408 of the input rotating shaft 418. Thus, the rotation of the transmission is transmitted to the rotor 405.
A thrust washer 413 is provided to contact one end surface of the rotor 405, and a spring 412 for biasing the rotor 405 toward the bearing 418 is provided on the opposite side of the rotor 405 with respect to the thrust washer 413. That is, the thrust washer 413 is biased by the spring 412 to surface-contact the end surface of the rotor 405, thereby applying a given braking torque to the rotor 405 as shown in FIG. 17, so as to prevent play rotation of the rotor 405 to be caused by vibration or the like under a stop or idling of the vehicle.
A magnetic induction device 410 such as a Hall device is provided in the current feeding portion 404 at a position near the rotor 405 so as to be opposed to an outer circumferential surface of the rotor 405. Further, a base 414 for receiving an electric signal to be generated from the magnetic induction device 410 is provided in the current feeding portion 404. The base 414 is connected to a lead wire 415 so as to output the electric signal through the lead wire 415 to an indicating instrument or the like (not shown).
In the rotation detecting device 401, the rotor 405 tends to be rotated in a small angular range because of play generated at a connecting portion between the rotational driving source of the transmission and the coupling. Occasionally, such play rotation of the rotor 405 causes a problem, such that although no rotation of the transmission is transmitted to the rotor 405, an electric signal indicative of rotation of the rotor 405 is erroneously output from the magnetic induction device 410. To avoid such a problem, a braking mechanism for preventing the play rotation of the rotor 405 is provided in the rotation detecting device 401. The braking mechanism is constructed of the thrust washer 413 for urging rotor 405 toward bearing 418 and spring 412 for biasing the thrust washer 413. The thrust washer 413 is biased to surface-contact the end surface of the rotor 405, so that a predetermined braking force to be obtained by multiplying the load of spring 412 by a coefficient of fiction of the thrust washer 413 is applied to the rotor 405, thus preventing malfunction of the rotation detecting device due to the play rotation of the rotor 405.
However, in the above-mentioned rotation detecting device, as the braking force is obtained by multiplying the load of the spring 412 by the coefficient of friction of the thrust washer 413, the braking force will be reduced as time proceeds because of settling, wearing, etc. of the spring 412. Moreover, as shown in FIG. 2, the degree of reduction in braking force is remarkably large in an initial stage. Thus, there is a problem in respect of durability of the device.
As shown in FIG. 8, the braking force to be obtained by the spring 412 is constant irrespective of a vehicle speed. On the other hand, as shown in FIG. 9, it is apparent that a sufficient braking torque is required only under a stop or low-speed running condition of the vehicle where the play rotation of the rotor 405 tends to occur and that so large a braking torque is not required under a high-speed running condition of the vehicle. Such an optimum braking characteristic as shown in FIG. 9 cannot be obtained in the above conventional rotation detecting device since the braking force is obtained by multiplying the load of the spring 412 by the coefficient of friction of the thrust washer 413, and it is constant irrespective of a vehicle speed. Furthermore, as the load of spring 412 is always applied to thrust washer 413, the durability is reduced.
Further, as an end surface of the spring 412 is inclined with respect to a biasing direction in general, the end surface of the spring 412 does not uniformly abut against the thrust washer 413. Accordingly, as shown in FIG. 18, a braking force is varied according to a circumferential position of the spring 412, resulting in fluctuation of the braking characteristic of the rotor 405.
Further, as the braking torque to be applied to the rotor 405 is obtained by the biasing force of the spring 412 only in a thrust direction of the rotor 405, an amount of wear of the bearing 418 due to the thrust load of the spring 412 becomes large in the case of driving the rotor 405 at high speeds, thus causing a reduction in durability of the rotation detecting device.